Very long cylindrical units, in particular lengths of telephone cables, are generally laid in position inside ducts by hauling one end of the unit by means of a rope hitched to a haulage device e.g. a winch or a capstan.
Said operation soon comes up against a main difficulty, namely the increase in the required haulage force to overcome friction between the hauled unit and the wall of a duct. Said haulage force can be greater than the breaking strength of the hauled unit.
For example, a currently made optical fibre cable cannot withstand a traction force of more than 130 kilograms weight and for haulage distances of about 600 m, the force necessary to haul a cable is 2000 kilograms weight.
Of course, shorter lengths of cables could be hauled, but in that case, a large number of splices are required along the line and that would cause an attenuation in transmission and a larger number of points of access to the duct would also be necessary.
Document FR-A No. 2 450 775 describes an installation which uses auxiliary devices disposed at points of access to a duct and which are driven by an electric motor and synchronized by a control panel connected to each of the devices by an electric cable thereby limiting the use of the auxiliary devices to short distances.
A unit can be driven along by rollers, caterpillar tracks or tyre-mounted wheels as described in document FR-A No. 2 259 460.
Preferred embodiments of the present invention make each auxiliary device placed along a duct independant and self-adjusting.
For cables, said auxiliary devices are generally placed at the existing bends in each duct, since said ducts rarely have rectilinear portions of great length.
Preferred embodiments of the invention also improve the accuracy of measurement of the traction force since in conventional haulage devices, the traction force is measured on the winch. This can give rise to errors of up to about 40%.